The invention concerns a phase plate for electron optical imaging, wherein the electron beam is split by an object into a zero beam and diffracted electron beams, wherein the phase plate is designed as an electrode for phase-shifting the zero beam using an electric field generated by a voltage, in order to obtain a high-contrast image through its interference with diffracted electron beams.
The invention also concerns an imaging method, wherein an image that is generated through electron optics and of which image information is lost due to partial shading of the optical path of the diffracted beams, is reconstructed by utilizing the image information that is identical to the lost image information and is located centrally symmetrically opposite to the zero beam.
The invention also concerns an electron microscope, wherein a phase plate of the above-mentioned type is disposed in the objective focal plane of the zero beam.
In electron microscopy, many objects are almost transparent to the beam, as in light microscopy, such that only little amplitude contrast is obtained with conventional imaging. Both types of beam are phase-shifted in a position dependent fashion during passage through the object in dependence on the structure of the object. The beam is thereby divided on the object into an undiffracted so-called zero beam and diffracted beams of several orders of diffraction.
When the zero beam is phase-shifted, preferably through 90°, the phase modulation of the object is changed into a strong amplitude contrast when the zero beam is again superposed with the diffracted beams in the image plane. This is the well-known phase contrast (see e.g. Reimer, Transmission Electron Microscopy, page 199 ff, Berlin, Heidelberg, New York 1984). The fact that the beams form different focal points in the focal plane of the objective is utilized for such a phase shift. The zero beam forms a central focal point in the focal plane for the zero beam and is surrounded by the focal points of the diffracted beams of the different orders of diffraction, starting with the diffraction of first order, which are substantially in the same focal plane. This fact can be utilized to either phase-shift the zero beam or the diffracted beams in order to eliminate the phase difference to increase the amplitudes. The invention is thereby based on a phase-shift of the zero beam.
In contrast to light optics, electron beams are disadvantageous in that there is no transparent plate that can carry a phase plate. In order to solve this problem, phase plates of the above-mentioned type were proposed which differ, however, with respect to the precise designs, as is explained below. With respect to all phase plates of this type, it must be noted that it is not technically possible to only shift the phase of the zero beam, since this would require an electric field in the nanometer range which cannot be generated with present technology. For this reason, depending on the design of the respective phase plate, at least a few low orders of diffraction also experience a phase shift. The term “phase plate” which is derived from light microscopy, is rather symbolically used in the field of electron microscopy, since it usually does not concern a plate-shaped component, and includes the components that are required to obtain the phase shift.
EP 0 782 170 A2 proposes a phase plate and an electron microscope, wherein a ring-shaped electrode is provided having a bore in the area of the zero beam, in which it generates the phase-shifting electric field. The ring-shaped electrode is connected at opposite sides to holding arms which extend to the housing of the electron microscope. A phase plate of this type has several disadvantages. Firstly, the ring that must have a certain size due to mechanical and production reasons, shields off beams in the vicinity of the zero beam. Since these are the diffracted beams of highest intensity, many beams for forming the contrast are lost. Moreover, the opening of the ring must be very small in order to prevent inclusion of an excessive number of low orders of diffraction for the phase shift, since these are then also lost for the formation of contrast. This, in turn, causes this small central bore to quickly soil and become clogged through contamination. Another disadvantage is that the holding arms for the ring extend through the optical path of the beams of higher order diffraction, with the consequence that shading also occurs and information is lost.
In order to solve the last-mentioned problem, DE 102 06 703 A1 proposes a phase plate and a method of the above-mentioned type, wherein the ring electrode is held by holding arms which are not centrally symmetrically disposed, such that those arms do not generate any shading of the kind caused by arms which are disposed at 180° opposite to each other with respect to the zero beam passing through the bore of the ring electrode. The shading of the holding arms can thereby be reconstructed through an imaging method of the above-mentioned type. The above-mentioned problems with the ring electrode remain, however, unsolved, in particular, their shading extends over the area of a ring that extends around the optical axis, which in this respect excludes any of the above-mentioned recombination possibilities.
DE 10 2005 040 267 A1 discloses a method for generating a phase plate of the above-mentioned type with a ring electrode, which is designed in layers like a sandwich and is then provided with the bore for the ring electrode. The latter, however, also has the above-mentioned shading.
JP 2006162805 A proposes preventing adhesion of protective particles on a phase plate through electrostatic forces. Towards this end, a corresponding layer design for a phase plate is proposed. This design of the phase plate, however, has nothing to do with the above-mentioned question of image recombination through centrally symmetrically opposite image information.
A further proposal according to EP 0 782 170 A2 avoids the ring structure of the electrode, which also eliminates the above-mentioned problems associated therewith. This proposal in accordance with FIG. 3 and the associated description corresponds to the phase plate of the above-mentioned type. As to the design of such an electrode, the only proposal consists in that a conductor is directly tangentially guided past the zero beam. In order to ensure that the electric field emitted by this conductor preferably only influences the zero beam, FIG. 3 and the associated description propose to surround it with an insulating layer and an earthed shielding layer which is removed from the periphery of the entire conductor in the area of the zero beam, such that an electric field is generated between the conductor and the two circular ring shaped ends of the shielding layers, which overlaps the zero beam on one side of the conductor. This eliminates the above-mentioned problems with the ring electrode, but the problem that the shielded conductor passes through the entire optical path of the diffracted beams and generates correspondingly large shadows remains. This conductor with insulation and shielding is essentially minimally offset with respect to the focal point of the zero beam, such that also in this case, the areas which are centrally symmetrically offset by 180° with respect to the focal point of the zero beam are shaded. Apart from the fact that EP 0 782 170 A2 does not propose recombination, such recombination would not be possible due to the above-mentioned fact. One further problem inherent with this proposal is the fact that the conductor also generates an electric field on the side facing away from the zero beam, since it is also exposed without shielding at that location. In consequence thereof, the diffracted beams that extend there are also phase-shifted and are therefore no longer available for increasing the amplitude.
DE 102 00 645A1 proposes providing an aperture collimator that bears an annular phase plate which extends along the collimator border in the form of a narrow edge and is designed e.g. as an annular electrode with an inner electrode and outer earth electrodes. Alternatively, this annular phase plate is inwardly offset from the aperture collimator and held by arms. In order to ensure that the zero beam reaches the area of the field of this electrode, the optical path is correspondingly tilted. This solves the problem that there is no small bore that could be soiled or clogged through contamination. On the other hand, it takes some effort to appropriately tilt the optical path about the exact angle. In the first embodiment, the aperture collimator cuts away more than half of the diffracted beams due to its arc shape, which excludes a combination between the proposed solution and the above-mentioned image reconstruction. In the second embodiment, the aperture collimator is outside of the optical path, but the annular field acts not only on the zero beam but also in arcs through the entire optical path of the different orders of diffraction to the edge of the optical path, as in the first embodiment. With this electric field design, the part of the diffracted beams, which is thereby influenced, can no longer be used to increase the amplitude due to the phase shift generated by the field. Nor is recombination of the image possible, since the field generates phase shift on areas which are 180° opposite to the zero beam.
It is therefore the underlying purpose of the invention to provide a phase plate and an electron microscope, wherein shading by the phase plate is reduced to a minimum and no shading occurs that cannot be reconstructed. With respect to the imaging method, the object consists in enabling complete recombination of the image.